Multipath laser moving target indicator

ABSTRACT

A moving target velocity indicator including a multipath laser which operates simultaneously as an oscillator for producing a laser beam which is directed onto both a moving target and a detector and as an amplifier of the Doppler-shifted laser beam reflected from the target. The optical return signal from the target is amplified while passing through the multipath laser system and is directed, after the final pass through the laser medium, onto the detector, This amplified signal can be combined at the detector with the transmitted signal to provide a beat signal at the frequency of the Doppler shift which can be applied to an oscilloscope display calibrated in frequency. Knowing the frequency of operation of the laser, the velocity of the target can then be calculated.

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States ieslmair et al.

MULTIPATH LASER MOVING TARGET INDICATOR Inventors: Hans Hieslmair,Elberon; John N. Fulton, West Long Branch; Charles J. Bickart, Ocean,all of NJ.

The United States of America as represented by the Secretary of theArmy, Washington, DC.

Filed: Apr. 2, 1971 Appl. No.: 130,740

Assignee:

US. Cl. ..356/28, 330/43, 331/945 Int. Cl ..G0lp 3/36, H015 3/00 Fieldof Search ..356/28; 331/945 A,

References Cited UNITED STATES PATENTS H.V SUPPLY FORElGN PATENTS ORAPPLICATIONS 1,183,492 3/1970 Great Britain ..356/28 PrimaryExaminerBenjamin A. Borchelt Assistant ExaminerS. C. BuczinskiAttorney-Harry Mv Saragovitz, Edward J. Kelly, Herbert Berl and DanielD. Sharp [57] ABSTRACT A moving target velocity indicator including amu]- tipath laser which operates simultaneously as an oscillator forproducing a laser beam which is directed onto both a moving target and adetector and as an amplifier of the Doppler-shifted laser beam reflectedfrom the target. The optical return signal from the tar get is amplifiedwhile passing through the multipath laser system and is directed, afterthe final pass through the laser medium, onto the detector, Thisamplified signal can be combined at the detector with the transmittedsignal to provide a beat signal at the frequency of the Doppler shiftwhich can be applied to an oscilloscope display calibrated in frequency.Knowing the frequency of operation of the laser, the velocity of thetarget can then be calculated.

5 Claims, 2 Drawing Figures PATENTEDHAY 8 1915 INVENTORS HANS H/EsLMA/R,CHARLES J, B/CKART a N. FULTON ATTORNEYS MULTIPATH LASER MOVING TARGETINDICATOR BACKGROUND OF THE INVENTION amplification and generationfunctions. Furthermore,

alignment problems become acute with such systems since the transmittedbeam and the return beam are traversing different optical paths; this isparticularly troublesome in view of the inherently narrow aperture ofoptical beams. In the past, multipath lasers have been used eithersolely as an amplifier or solely as an oscillator, with the oscillatorhaving the usual cavitydefining mirrors at each end of the laser medium.Such an arrangement could not be suitable for target information systemssince the cavity mirror would attenuate the return signal, which, inmost instances, already is quite weak owing to the distance factor andto scattering of the beam upon reflection from the target. Furthermore,the use of a cavity would greatly restrict the oscillator bandwidth. Inorder to obtain satisfactory operation of target information systemscharacterized by return signals of very small magnitude, a considerablyhigher gain is required than can normally be achieved by an ordinarylaser amplifier. This gain can be increased considerably by using amultipath laser amplifier; however, prior use of such devices has beenrestricted to amplifier operation. The present invention, which uses amultipath laser wherein the pump power is increased to just above thethreshold for selfoscillation, so that it can operate simultaneously asan oscillator and amplifier, forms part of a novel Doppler moving targetindicator system differing from conventional Doppler system in which thereturn signal from the moving target arrives at the detector directly.The system of the present invention, in addition to requiring a lowerlaser beam power output for the same detectability, has the advantageover a system using a separate amplifier and oscillator, of a greatlyreduced alignment problem since the output laser beam and the returnbeam signal of inherently narrow beam width travel through a commonoptical path.

SUMMARY OF THE INVENTION The system of the invention uses a gas laser,such as a carbon dioxide laser, which includes at one end an initiallyadjustable reflector or mirror which may be fixed during actualoperation and a pair of closely spaced reflectors at the other end whichcan be adjusted relative to one another and to the fixed reflector atthe other end. The laser beam is reflected back and forth between firstone of the two adjustable end reflectors and the fixed reflector andthen between the other of the two adjustable end reflectors and saidfixed reflector, thereby passing back and forth through the gaseouslaser medium several times, each time over a somewhat different path. Bysimple adjustment of one or both of these reflectors, the number oftimes that the laser beam traverses the gaseous laser medium can bevaried and the gain of the laser, which is dependent upon the number ofsuch traversals, is adjustable. A portion of the generated laser beamtraversing the initial beam path passes through one of the outputwindows enroute to a moving target. Another portion of the beamtraverses the final beam path and passes through another laser outputwindow to a suitable detector and display. The relatively weak returnsignal from the target, which is shifted in frequency in accordance withthe wellknown Doppler effect, returns along the same path over which thelaser beam was propagated and is amplified as it negotiates the multiplepasses through the laser medium until it is directed along said finalbeam path onto the detector. This amplified return signal is combined atthe detector with the transmitted laser beam to provide a beat signal atthe frequency of the Doppler shift which can be applied to a displaymeans such as an oscilloscope calibrated in frequency. The velocity v ofthe target relative to the observer can then be calculated from therelation v )tAfo/g where A is the known wavelength of operation of thelaser and AfD is the beat frequency, which can be read from thecalibrated display.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram, partlyin section, of a moving target detector system according to theinvention; and

FIG. 2 is a detailed view, partly in section, showing details ofadjustment of the adjustable reflectors at one end of the laser tube ofFIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The moving targetindicator system of FIG. 1 includes a multipath gas laser device 10having an envelope 12 for containing a gaseous lasing medium, such ascarbon dioxide, usually combined in smaller amounts of such gases asnitrogen and helium. The laser 10 is pumped by direct current excitationof the gaseous medium using a high voltage direct current supply 14connected to electrodes 15 mounted in projections 17 which extend offaxis of the laser tube envelope 12 so as to avoid obstruction of thelaser beam paths through the laser envelope and to trap any materialsputtered or otherwise emitted from the electrodes which would causecontamination of portion 18 of the envelope 12 which, for example, maybe pyrex glass tubing. A resistor 20 is provided in the pumping circuitand serves as a discharge load for minimizing transient surges duringfiring of the laser 10. Although not shown in the drawing, moreefficient depopulation of the lower laser level of the CO molecule canbe achieved, and more adequate cooling at higher power levels can beprovided, by means of a water jacket surrounding the tube envelope. Themultipath optics for the laser 10 includes three reflectors of equalspherical radius of curvature. Two semicircular reflectors 22 and 23 areset close together at one end of the laser and a single reflector 24 ismounted at the other end. The center of curvature of the split reflectormade up of reflectors 22 and 23 is on the front surface of reflector 24and the center of curvature of reflector 241 is disposed halfway betweenreflectors 22 and 23; with this arrangement, a system of conjugate fociis established on the reflecting surfaces of the reflectors. Thereflectors 22 and 23 are each adjustable by means of offset adjustingscrews 26 to 29 which threadably engage an end plate fastened to theportion 19 of envelope 12 by screws 32. The metal portion 19 of theenvelope is hermetically sealed to the glass portion 12 of the envelopeby appropriate glass-to-metal sealing techniques. A hermetic seal foreach of the adjustable screws is accomplished by means of the bellows35. The reflectors 22 and 23 are adapted to rotate about a vertical axisby means of respective screws 27 and 28 and are adjustable about ahorizontal axis by means of the corresponding adjustment screws 26 and29. Although the reflector 24 is shown in the drawing, for the sake ofsimplicity, as fixed, this reflector 24 can also be made adjustable inboth the horizontal and vertical planes by similar adjusting screws.However, the reflector 24 need be adjusted only initially to assist inorienting the laser beam so that it passes through the opticallytransparent Brewster windows 36 and 37, and once properly set, is leftin that position during subsequent laser operation. By adjustingreflectors 22 and 23 relative to one another and to reflector 24, thenumber of paths or traversals of the amplified laser beam can be varied.The gain of the amplifier thus can be increased by increasing the numberof such laser beam passes through the gaseous laser medium. In order forthe laser 10 to operate as an oscillator, in addition to operating as anamplifier, the gain must be increased until the laser oscillationthreshold is attained. This can be done either by increasing the pumpingvoltage of the high voltage supply 14 or by adjusting the reflectors 22and 23 to increase the number of beam passes, or both. A simplifiedrepresentation of a compound beam path in the laser is shown in FIG. 1.As shown in FIG. 1, the laser beam, after passing through Brewsterwindow 36, passes through transmit-receive optics 40 to the target 45.At the same time, the laser beam is propagated through Brewster window37 onto a detector by way of the tilted mirror 51 and lens 52. Themirror 51 serves to permit lateral displacement of the detector 50 awayfrom the transmit-receive optics 40 and the laser device, and the lens52 is used for focusing the return beam upon the detector 50. A portionof the laser beam, upon striking the target 55, is reflected therefrom.Since a certain amount of scattering occurs during this restrictionprocess and also during return through the atmosphere to the laser, itis necessary, in practice, to use the transmit-receive optics 40 toprovide a large aperture for collecting the relatively weak returnsignal and focus it back by a system of reflectors 55 and 56 and thencethrough the aperture 57 along the path over which the beam originallywas launched. After making the several beam passes, shown in FIG. 1 asbeing eight passes, the return beam or signal passes through theBrewster window 36 and is supplied to the detector 50. The detector 50,which for example, may be mercury-cadmium telluride or gold-dopedgermanium, is preferably mounted in a dewar 59 for achieving the usualreduction in signal-to-noise ratio accompanying low temperatureoperation. The beat frequency signal from the detector 50 is amplifiedby an amplifier 60 and applied to an oscillator display 62 or othersuitable display means. As the system operates in the continuous wavemode, the frequency f of the laser oscillator energy transmitted to thetarget 45 will be shifted to f +Af where Af is the Doppler shiftresulting from movement of the target relative to the detector. The

detector 50 will respond to the frequency f of the laser oscillator beamsupplied by way of window 37 and to the return signal of frequency f +Afand the output of the detector will be the beat frequency, viz.,f,, (f Af,,) Af and the oscilloscope display will appear as a sinusoidal wavewhose frequency can be picked off directly from the calibratedoscilloscope by simply counting the spacing between successive cycles onthe oscilloscope screen. Since the beat frequency A f is given by andsince A is the reciprocal off and is known for the given laser, one cancalculate the relative target velocity v as the ratio of the Dopplerfrequency shift A f to twice the laser frequency.

While the invention has been described in connec- :ion with anillustrative embodiment, obvious modificaiions thereof are possiblewithout departing from the spirit of the invention. Accordingly, theinvention should be limited only by the scope of the appended claims.

What is claimed is:

l. A moving target indicator system comprising a laseroscillator-amplifier device including a lasing medium and pumping meansfor supplying pump energy to said lasing medium, multi-path opticalmeans comprising reflector means spaced at opposite ends of said lasingmedium for directing a generated laser beam through said lasing mediumseveral times along a plurality of paths, said pump energy and saidmulti-path optical means contributing to provide a laser gain sufficientto reach a self-oscillation threshold, said laser device includingoptically transparent means independent of said multi-path opticalmeans, a detector, said multi-path optical means further directing saidlaser beams through said optically transparent means onto a target andonto said detector, said multi-path optical means further directing ontosaid detector the return laser beam from said target received by way ofsaid optically transparent means along said plurality of paths to effectsubstantial amplification of said return beam, said detector producing abeat frequency signal equal to the doppler frequency shift resultingfrom motion of said target.

2. A moving target indicator system according to claim 1 wherein theportion of the reflector means disposed at one end of said lasing mediumis adjustable to vary the number of paths of said laser beam throughsaid lasing medium.

3. A moving target indicator system according to claim 2 wherein saidadjustable portion of the reflector means comprises two semiconductormirrors which are independently adjustable about two orthogonal axes.

4. A moving target indicator system according to claim 1 wherein saidoptically transparent means includes a pair of output windows throughwhich said laser beam passes and disposed outside the region occupied bysaid reflector means.

5. A moving target indicator system according to claim 2 wherein saidoptically transparent means includes a pair of output windows throughwhich said laser beam passes and disposed outside the region occupied bysaid reflector means.

1. A moving target indicator system comprising a laseroscillator-amplifier device including a lasing medium and pumping meansfor supplying pump energy to said lasing medium, multi-path opticalmeans comprising reflector means spaced at opposite ends of said lasingmedium for directing a generated laser beam through said lasing mediumseveral times along a plurality of paths, said pump energy and saidmulti-path optical means contributing to provide a laser gain sufficientto reach a selfoscillation threshold, said laser device includingoptically transparent means independent of said multi-path opticalmeans, a detector, said multi-path optical means further directing saidlaser beams through said optically transparent means onto a target andonto said detector, said multi-path optical means further directing ontosaid detector the return laser beam from said target received by way ofsaid optically transparent means along said plurality of paths to effectsubstantial amplification of said return beam, said detector producing abeat frequency signal equal to the doppler frequency shift resultingfrom motion of said target.
 2. A moving target indicator systemaccording to claim 1 wherein the portion of the reflector means disposedat one end of said lasing medium is adjustable to vary the number ofpaths of said laser beam through said lasing medium.
 3. A moving targetindicator system according to claim 2 wherein said adjustable portion ofthe reflector means comprises two semiconductor mirrors which areindependently adjustable about two orthogonal axes.
 4. A moving targetindicator system according to claim 1 wherein said optically transparentmeans includes a pair of output windows through which said laser beampasses and disposed outside the region occupied by said reflector means.5. A moving target indicator system according to claim 2 wherein saidoptically transparent means includes a pair of output windows throughwhich said laser beam passes and disposed outside the region occupied bysaid reflector means.